Unauthorized entry into systems that provide privileged information or services is a security problem that has grown considerably since the dawn of the computer age. For instance, the telephone service industry loses approximately two billion dollars a year because of unauthorized entry into its subscriber services. The cellular phone industry has typically responded to telephone fraud by providing each authorized user with an access code to be transmitted to the phone service to gain entry. However, an intercepting apparatus costing as little as $250 has recently emerged in California which electromagnetically intercepts and identifies transmitted access codes. Once an access code is intercepted, an unauthorized user can typically charge telephone calls to the authorized user's account until the fraud is detected when the authorized user receives his telephone account bill--typically at the end of each month. Once notified of the fraud, the telephone service will swiftly change the access code and most likely bear the cost of the fraudulent calls.
Telephone fraud problems also arise with telephone credit cards which provide an access code for charging a telephone service subscriber's long distance telephone calls. A typical scam involves a spy using binoculars or like device to visually intercept an access code that is being entered into a pay telephone by an authorized user. Once the access code is intercepted, the stolen code can be repeatedly used by the interceptor until the authorized user learns of the fraud when receiving his periodic telephone service bill.
Telephone services have attempted to make it more difficult to steal a telephone service access code. For example, U.S. Pat. No. 4,761,808 issued to Howard shows a time code telephone security access system. The system uses an access code comprising an address code that is fixed and a time-based code portion that changes over time in a predetermined manner. However synchronization problems can easily arise between the transmitting end and the receiving end of a telephone call using such a system. For example, some cellular phones include an internal clock for determining the time-based access code. If the clock in the cellular phone is not perfectly synchronized with the clock at the telephone service, a telephone call by an authorized user may be rejected. Similar problems arise when using a telephone credit card if the timepiece of the authorized user is not synchronized with the clock of the telephone service.
It is the object of the present invention to overcome the drawbacks and disadvantages of prior art apparatuses and methods for preventing access to a telephone service.